I think you can't expect sudden surge in your verbal score w/i short period of time. A lot of reading, and memorizing vocabs on a daily basis, then reviewing those vocabs you have memorized.
I am an international student, so this task is probably less tedious and repetitive than native speakers..
Some of the books I have found useful are
1. Word Power Made Easy by Norman Lewis.
-This book is some decades old I think. But I find it very very help in
learning roots. It describes each root and words and shows usage as well.
I went through it twice, and plan to go over it 2~3 more times.
2. Word Smart I,II by Princeton Review
These books are somewhat like simpler version of Word Power Made Easy.
They don't really introduce roots or prefixes, but they provide simple
definitions and usage of each word.
I have heard that they have Word Smart GRE as well.
3. Reading TIMES, and National Geographic. (or other magazines if you have
time). I find reading TIMES very helpful..Whatever words I don't know I
would highlight and look it up later and keep track of them.
Reading some sociology books or classic novel is a great means to improve
your vocab.
When studying vocab, I tried to be active. Not only would I try to memorize the exact definition of words, but also, I tried to make a short sentences out of them, and think of in which situation I'd use them. I say them out loud, write them down a few times. Once I "studied" the vocabs, then I just read through the definitions periodically so I don't forget them.
Right now, I can afford time to do these all these. But once I am in school I plan to just go through 10~15 vocabs a day and review them at the end of each week. Reviewing is the most important thing.